The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for frequency control. Merely by way of example, the invention has been applied to a power converter. But it would be recognized that the invention has a much broader range of applicability.
Power converters are widely used for consumer electronics such as portable devices. The power converters can convert electric power from one form to another form. As an example, the electric power is transformed from alternate current (AC) to direct current (DC), from DC to AC, from AC to AC, or from DC to DC. Additionally, the power converters can convert the electric power from one voltage level to another voltage level. The power converters include linear converters and switch-mode converters.
The switch-mode converters can consume significant power under standby conditions. For example, the standby conditions correspond to light or zero output loads. The standby power includes energy losses at various components of the switch-mode converters such as power switches, transformers, inductors, and snubbers. These losses often increase with the switching frequency. To reduce the standby power, the switching frequency is usually lowered for light or zero output loads. FIG. 1 is a conventional diagram for switching frequency control. When the control voltage is lower than a threshold voltage, the switching frequency is smaller than the normal frequency. The control voltage increases with the output load.
Some conventional switch-mode converters use an oscillator to generate a switching frequency. The switching frequency is modulated based on the magnitude of an output load. For example, the oscillator includes a ramping waveform generator or an RC oscillator if the normal frequency is in the range of tens of kHz. The rising time is controlled by a current source to charge a capacitor, and the falling time is controlled by a current sink to discharge the capacitor. By controlling the charge and/or discharge current of the capacitor, the switching frequency is modulated. But this conventional technique often cannot provide sufficient stability to the switch-mode converters.
Hence it is highly desirable to improve techniques for switching frequency control.